Article storage facilities, in which articles (which may be article containers for storing or holding articles) can be automatically carried in (i.e., stored) and carried out (i.e., retrieved), are widely in use. Such article storage facilities often have a plurality of levels of article storage shelves, that are spaced apart from one another, each of which can store a plurality of articles in the horizontal direction. As for how the articles are carried into and out of such an article storage facility, there is a method (for example, method that utilizes horizontally moving carriages) in which carrying in and out of articles along the horizontal direction and carrying in and out of articles along the vertical direction are performed separately. And there is also a method (for example, method that utilizes a stacker crane) in which carrying in and out of articles along the horizontal direction and the vertical direction is performed together. In the method that utilizes horizontally moving carriages, for example, carriages, one of which is independently provided for each level and which can move horizontally on the corresponding level, perform the carrying in and out of articles to and from the respective article storage shelves on respective levels. And articles are carried in and out of the article storage facility by causing a lift, which is provided to be used by all levels and which can be moved vertically, to transfer articles that are carried into and out of each level.
JP Publication of Application No. H07-304508 (Patent Document 1) discloses an article storage facility (automatic storage and retrieval device) of the type that utilizes horizontally moving carriages. This article storage facility is configured to be able to perform storage and search/retrieval of articles automatically using carriages called pickers (inner pickers). The picker that is associated with, or provided for, the article storage shelf of each level travels on a track provided along the open side of the corresponding article storage shelf to carry in and out an article to and from the article storage shelf. First optical communication unit is provided in the travel space above the track such that the unit faces the picker whereas the picker is provided with second optical communication unit. Light signal is exchanged between the first optical communication unit and the second optical communication unit. And the picker carries an article into and out of the article storage shelf and transports an article along the track based on the control signal transmitted from the first optical communication unit.
Incidentally, as described above, when there are two or more vertical levels of article storage shelves, light, which is a medium which transmits signals in optical communication, spreads or becomes diffused, making it possible for the light to reach an article storage shelf at a different level from the article storage shelf that is the intended target of the optical communication. For example, when light signals are transmitted from a plurality of first optical communication units in the same period, the light signals may interfere with one another, making it possible for the picker that received the light signals through the second optical communication unit to malfunction or perform a wrong operation. To this end, in Patent Document 1, the first optical communication unit is mounted above the track at one end of the travel space for the picker. The light signal transmitted from the first optical communication unit of a particular level is blocked by the tracks on the level above and below that particular level, making it difficult for the light signal to reach a picker that may exist on the track above or below the particular level. Therefore, even when there are two or more vertical levels of article storage shelves, interference between the light signals can be alleviated, thus reducing the possibility that a picker may malfunction or perform a wrong operation as described above.
The technology of Patent Document 1 is advantageous in that interference between the light signals is alleviated, and the possibility that a picker may malfunction or perform a wrong operation is reduced. However, since the first optical communication units are installed on the tracks, there is a certain constraint placed on the structure of the facility. Therefore, a technology is also desired which is adapted for wider usage and in which interference between the light signals during optical communication is reduced irrespective of the structure of the facility.